This invention relates to a metal tube, and more particularly to a metal tube having a corrosion resistant powder coated outer surface.
Metal pipes often have their outer surfaces covered with a protective coating. These pipes are used for conveying brake fluids, fuel and the like in a motor vehicle. As such, these pipe lines are located under the body of the vehicle. Since they are used in such a harsh environment, the pipes are required to have a high degree of corrosion resistance, scratch resistance, impact strength and mechanical wear resistance. In cold climates, it is not unusual to encounter road salt sprinkled onto road surfaces in order to prevent icing on the road surfaces and the inherent dangers caused thereby. The popularity of spreading road salt has created a serious problem of metal pipe corrosion. The pipes are also vulnerable to damage or wear from stones or mud spattered by rotating wheels of the vehicle. Therefore, it is necessary that the pipes attached to the underbody of the vehicle be coated so as to resist both chemical corrosion and mechanical damage or wear.
A double-rolled steel pipe has been proposed that is made by rolling a steel strip or hoop twice and brazing its longitudinal edges by means of a copper plating layer, or a seam welded steel pipe, where the pipe has an outer surface coated with an electroplated zinc film. The zinc film has an outer surface coated with a relatively thin special chromate film having, for example, an olive color. Chromate is typically used to enhance the adhesion of the subsequent layer by making the surface rougher. The chromate film has an outer surface coated with a fluorinated resin film. The fluorinated resin film is formed by impregnating the chromate film with a dispersion of polyvinyl fluoride immediately after the formation of the chromate film when it is still in the state of a gel, and drying them under heat, so that the fluorinated resin film may form an intimate bond with the chromate film. When the chromate film is formed by treating the pipe with a solution, it requires large amounts of a chromium compound and an organic acid, such as formic acid, used as a reducing agent. It is necessary to supply the treating solution with the chromium compound frequently, and to renew it at regular intervals of time in order to maintain a constant film forming capacity. The waste solution, however, contains a large amount of chromium having a valence of 6, which is a toxic substance. As such, the disposal of this toxic waste solution is very costly. Although the chromate film as formed is highly resistant to corrosion, the heat to which it is exposed during the formation of the resin film deprives it of water and thereby makes it brittle. Any plastic deformation of the pipe, such as may result from bending or double flaring, forms fine cracks in the chromate film which lowers its rustproofing properties.
It has also been proposed to provide a corrosion resistant pipe where a metal pipe is provided with an outer surface coated with a zinc film, a chromate film, an intermediate layer consisting sequentially of an epoxy resin and a polyvinyl fluoride film formed one on top of another in the order listed.
A plastic-coated steel tube has also been proposed where a steel tube has an inner layer of at least one cross-linked polyolefin modified with a hydrolyzable silane and an outer unmodified or soot-blended polyolefin layer on the exposed surface of the inner layer.
A process for coating metal tubes with plastic material has also been disclosed where a fixed metal tube is heated to a temperature above the melting point of the plastic material to be employed, thereafter causing a mixture of plastic powder and air to pass through the metal tube whereby the plastic material is fritted onto the inside surface of the tube, thereafter rotating the metal tube and applying to the exterior surface thereof in a plurality of stages a plastic material, the plastic material being electrostatically sprayed onto the rotating metal tube. After each stage of electrostatically applying plastic to the outside surface of the metal tube and applying plastic material to the inside surface thereof, the plastic material is completely melted and smoothed. However, this process has several drawbacks, including that it is a batch process (as opposed to a continuous process); it requires rotation of the tube; and the cure time of the plastic material is quite slow, eg. several minutes. Thus, this process is rather inefficient, both from a cost and time standpoint.
An automobile tube line for a brake, fuel or hydraulic system has also been disclosed with an interior steel tube having a galvanized exterior layer with an additional exterior olive chromated layer which is wrapped in an additional Nylon 12 layer casing where the plastic casing is a polyamide layer applied by extrusion on top of the olive chromated layer.
A conventional method of applying a polyvinyl fluoride (PVF) or polyvinylidene fluoride (PVDF) coating onto tubing is by applying the PVF or PVDF as a paint. As such, the PVF or PVDF is carried in a solvent to form the paint. As the solvent evaporates, the paint dries, thereby leaving the PVF or PVDF coating on the tube. This is not an environmentally "friendly" practice, as much solvent vapor may be generated. Thus, complex and costly equipment is necessary for vapor recovery and post treatment.
Thus, it is an object of the present invention to provide a corrosion resistant metal tube and process for making the same which advantageously substantially eliminates volatile organic compounds, thereby substantially eliminating the need for vapor recovery. It is a further object of the present invention to provide a process which utilizes nearly 100% of the material applied to the tube, thereby advantageously substantially eliminating waste of raw material(s). Still further, it is an object of the present invention to provide such a tube and process for making the same which exhibits good adherence between the tube and a subsequent corrosion resistant layer(s). Yet further, it is an object of the present invention to provide a metal tube and process for making the same which advantageously substantially eliminates the use of chromium, thereby advantageously substantially eliminating the need for waste removal of this toxic substance. Further, it is an object of the present invention to provide a process for making such a tube which is continuous, and advantageously has a fast cure time and is cost effective.